The Earth is constantly changing shape. To be understood in context, when the motion of the Earth's crust is observed, it must be referenced. A Terrestrial Reference frame provides a set of coordinates of some points located on the Earth's surface. It can be used to measure plate tectonics, regional subsidence or loading [1] and/or used to represent the Earth when measuring its rotation in space. This rotation is measured with respect to a frame tied to stellar objects, called a celestial reference frame. The International Earth Rotation and Reference Systems Service (IERS) was created in 1988 to establish and maintain a Celestial Reference Frame, the ICRF, a Terrestrial Reference Frame, the ITRF. The Earth Orientation Parameters (EOPs) connect these two frames together. These frames provide a common reference to compare observations and results from different locations [1]. Nowadays, four main geodetic techniques are used to compute accurate coordinates: the GPS, VLBI, SLR, and DORIS. Since the tracking network equipped with the instruments of those techniques is evolving and the period of data available increases with time, the ITRF is constantly being updated. 11 realizations of the ITRS were set up from 1988. The latest is the ITRF2005. All these realizations include station positions and velocities. They model secular Earth’s crust changes that’s why they can be used to compare observations from different epochs. All the higher frequencies of the station displacements can be accessed with the IERS conventions, chapter 7 [2]. Continuity between the realizations has been ensured as much as possible when adopting conventions for ITRF definitions. The relationship linking all these solutions is of utmost importance. They are supplied here by the transformation parameters.

The International Terrestrial Reference System (ITRS) is a world spatial reference system co-rotating with the Earth in its diurnal motion in space. The IERS, in charge of providing global references to the astronomical, geodetic and geophysical communities, supervises the realization of the ITRS. Realizations of the ITRS are produced by the IERS ITRS Product Center (ITRS-PC) under the name International Terrestrial Reference Frames (ITRF). ITRF coordinates were obtained by combination of individual TRF solutions computed by IERS analysis centers using the observations of Space Geodesy techniques : GPS , VLBI , SLR, LLR and DORIS. They all use networks of stations located on sites covering the whole Earth.

For more details:

    Terrestrial Reference System and Terrestrial Refrence Frame definition. TRS & TRF

    Relationship between TRS's , Terrestrial Reference Systems.

    ITRS & ITRF : The International Reference System/Frame.